Electrical circuit damping



96L 1941. A. w. BARBER 2,258,877

ELECTRICAL C IRCUIT DAMP ING Filed Jan. 26, 1939 SIGNAL UTILIZATION SOURCE MEANS y- R.F'.AMP. I.F.HMP.@ 25 37 I 1ST. DET. FDR-SUPPLY .4 395 35/7 Q r M 33 e SIGNAL. 9 UTILIZATION SOURCE :/O MEANS WENTOI-L Patented Oct. 14, 1941 UNITED s arr T FFICE 6 Claims.

My present invention concerns electrical circuit damping and in particular methods of and means for critically damping normally oscillatory circuits.

One object of my present invention is to provide critical damping in a circuit in which the normal response is oscillatory.

Another object of my present invention is to provide the circuit damping periodically at a rate higher than any normal response of the circuit being operated upon.

Still another object is to electrically prevent transientsin circuits in which it is desired to keep the normal damping low.

A still further object is to prevent transients in various circuits and at the same time to maintain a high gain or efficiency in such circuits.

A particular object is to provide circuit damping by means of a thermionic vacuum tube which applies a quenching signal to the circuit.

Another particular object is to apply the quenching to the audio frequency amplifier of a radio receiver and to derive the quenching signal from the radio or intermediate frequency amplifier of the receiver. 7

till another particular object is to utilize a single thermionic vacuum tube for amplifying signals and at the same time for quenching transients in the tube load circuit.

These and other objects will be evident from the detailed description of the figures of the drawing.

Many circuits used in communication systems involve capacity and inductance with less than critical damping. In audio or video frequency compensating circuits, transformers and filters the damping is almost always less than critical. When sharp nonperiodic waves strike these circuits oscillatory responses are set up. These oscillatory responses represent new and undesired signals generated within the system. The conditions become more favorable when tetrode and pentode vacuum tubes are used since the plate resistances of these tubes is high and they add little to the normal circuit damping,

I have discovered that damping may be applied periodically with an improvement in overall efficiency and fidelity of response over circuits in which passive damping such as a fixed resistor is used. Essentially my new method consists in shunting the plate circuit of a vacuum tube across the circuit to be damped and in applying a high frequency signal to the grid of the tube. In this way the circuit is brought to rest at a high rate and transients do not chance to build up. These transients are in eifect a free swinging of the circuit while the high frequency damping has the effect of bringing the circuit to rest at a high rate so that as soon as an impulse which might cause the circuit toswing is over the circuit responds in a normal way as if the impulse never existed. Thus transients are prevented and by using'a frequency higher than the normal response range of the circuit for damping the normal response of the circuit is not substantially afiected.

In applying my new system to a radio receiver, I have found that the signal in the radio or intermediate frequency amplifier forms a convenient source of high frequency damping tube driving signal. Similar arrangements may be used in television receivers. A further simplification I have found possible in which a single vacuum tube both amplifies and clamps the circuit. This is accomplished by applying both the signal to be amplified and a damping high frequency signal to a control element or control elements of a single vacuum tube.

In the drawing:

Fig. 1 shows a fundamental circuit embodying my present invention.

Fig. 2 shows my present invention applied to a radio receiver. 1

Fig. 3 shows a modified form of my present invention.

Fig. 4 shows various signal waves useful in explaining the operation of my present invention In Fig. l is shown the fundamental circuit of my novel damping system. A signal source i drives grid 3 of the Vacuum tube 2. Tube 2 may be a pentode including cathode 4, control grid 3, screen grid 5, suppression grid 6 and plate although other tubes such as triodes or tetrodes may be used. Cathode 4 is heated by conventional means not shown. Screen grid 5 is energized from a tap on the direct current voltage supply 12 and is by-passed by condenser 8. Tube 2 is intended to amplify signals from signal source I with a peak at some frequency. The amplification and peaking are determined by the parallel tuned circuit including inductance 9 having the series resistance H3 in parallel with condenser ll and connected in series between plate I and direct current source l2. Source I2 is bypassed by condenser !3. The amplified signals across circuit 9, 50, H are fed to utilization means 54 which is not limited to any particular device but it is desirable that its input should be a high impedance so that the characteristics of have a 55 circuit 9, lil, ll will not be greatly affected. By

way of illustration the signal from source I may have the wave-form shown at a, in Fig. 4. The resonated load circuit of Fig. 1 will be shock excited by the sharp wave-front and will generate a superimposed damped oscillation as shown at b in Fig. 4. In Fig. 1 the vacuum tube I5 is used to damp these oscillations by connecting its plate I6 to the high side of load 9, III, II, its cathode to ground G or a source of bias Voltage and its grid I! to a source of high frequency periodic voltage I9. In this Way the plate resistance of tube I5 is raised and lowered at a high periodic rate. Each time the plate resistance is driven to a low value circuit 9, I0, II is more than critically damped and free oscillations are quenched. Thus as soon as the sharp wave-form has passed oscillations which it generates are damped out and the circuit responds to the remainder of the wave-form as though no sharp impulse has passed. The current wave produced by the signal from I and the quenching signal from I9 is shown at c in Fig. 4. However, since the quench frequency is higher than the response of circuit 9, I0, II the resulting voltage passed on to utilization means I4 will be as shown at d in Fig. 4 in which the peaked wave-form shows the desired accentuation of high frequency components but undesired oscillations have been eliminated.

This system may be applied to many different systems such as sound amplifiers, radio, facsimile, or television receivers, etc. While in no way intended to limit the applications anticipated, I have found for instance that if the Wave-form of Fig. 4 has a duration of ,430 of a second and circuit 9, II], I I is peaked at 600 cycles that source I9 may have a frequency of 10,000 cycles per second or more. Very high frequencies may also be used for source I9 as for instance 500,000 cycles or more. In some cases a frequency of the order of 10,000 cycles might not be appreciably responded to by circuit 9, I0, II but utilization means I9 might be too responsive to frequencies of this order in which event much higher quench frequencies are recommended.

Fig. 2 shows my novel quenching system incorporated in a radio receiver of the superheterodyne type. A radio frequency and first detector unit 29 is connected to an antenna A and ground G and feeds an intermediate frequency amplifier 25. Unit ZI also includes a power supply. Plate 22 of the final tube of the intermediate frequency amplifier feeds an output coil 23 tuned to the intermediate frequency by condenser 24. Coil 23 is inductively coupled to a second detector input coil 25 tuned by condenser 25. The second detector'is diode 29 and is coupled to coil 25 by direct connection to plate 30 and by indirect connection thru load resistor 21 bypassed by condenser 28 to cathode 3I. Cathode 3! is grounded at G. Rectified currents flowing in load resistor 12"! set up demodulated voltages which are applied. to grid 3'! of audio amplifier tube 3 thru coupling condenser 32 and across a variable portion of volume control potentiometer 33, Audio amplifier tube 34 includes a cathode 38 furnished with a bias thru resistor 39 bypassed by condenser 49, control grid 31, screen grid 35 and plate 35. Plate 35 feeds an output transformer primary 4-6 and receives plate voltage from a positive potential point in unit 2|. Primary i6 is magnetically coupled to secondary 41 which feeds speaker 48.

As so far described, this receiver is conventional. The output tube, transformer and speaker generate transient oscillations due to distributed and stray capacity in the transformer and reflected motional impedance from the speaker. In order to suppress these transients, quench tube I5 is connected in the system with its plate I6 connected to plate 35 of the audio output tube, its cathode is connected to ground and its grid I! connected to the high side of second detector input coil 25 thru a blocking condenser 45. The intermediate frequency signals across coil 25 act as the desired high frequency quenching signal. Switch 4| is included which in one position shorts condenser 45 by making contact to point 42 and when in another position connects grid I! to a grid leak 44 by making contact to point 43. When condenser 45 is shorted, rectified voltage is applied to grid I? which applies a greater quenching action on positive modulation frequency peaks than on negative which is useful in that a high mutual conductance tetrode output tube may have a tendency to oscillate on positive grid peaks. This tendency to oscillate in the tube is effectively compensated by the increased quench action of tube I5. If quench action is desired independent of modulation percentage switch GI is thrown to point 43 and demodulation voltages are filtered from grid I? only intermediate frequency voltage being applied.

The quenching system just described damps more strongly on strong intermediate frequency signals although in an automatic volume control receiver only very weak signals reach coil 25 with greatly reduced amplitude yielding reduced quenching action. Since increased quenching is usually desirable on strong signals this mode of operation is very satisfactory and even desirable. While the system has been shown applied to a radio sound receiver it is equally effective on other types of receivers such as telegraph, facsimile and television in overcoming undesired transient responses.

Fig. 3 shows a simplification over the fundamental circuit of Fig. 1. Signals from a source I are applied to control grid 52 of an amplifier tube 50 thru high frequency choke 49. Tube 50 includes a cathode 56 receiving a bias from cathode resistor 54 by-passed by condenser 55, control grid 52, screen-grid 53 and plate 5I. Plate 5! is loaded by a tuned system including coil 9 having resistance It shunted by condenser II and is energized from a direct current source I2. Screen grid 53 is also energized from a point on source I2. Quenching'of the plate load circuit is accomplished by means of the amplifier tube 50 itself by applying the high frequency quenching voltage I9 to control grid 52. The high frequency choke 49 is used to prevent the shunting of high frequency source I9 by signal source I. The output across load 9, I 0, II is fed to a utilization means I4. The action of this single tube system is similar to that of Fig. 1 in that the plate resistance of tube 50 is made low at a high rate by driving source I9 damping out undesired transients in circuit 9, I0, II.

High frequency source I9 has not been shown in detail since it will readily be understood that any convenient high frequency source may be used. A thermionic vacuum tube oscillator is a convenient device for source I9. In general, only a few volts output is needed depending on the characteristics of tubes I5 or 59. The frequency of source I9 desired will depend on the system in which it is to be used. In sound, telegraph and facsimile systems, source I9 will be usually from 10,000 to 500,000 cycles while for television it may be of the order of 5,000,000 cycles and up. Television intermediate frequency voltage having a mean frequency of the order of 12,000,000 may be convenient.

Whil many sharp wave-fronts exist in sound telegraph, facsimile and television signals are made up largely of sharp wave-front signals.

Thus my novel damping system is desirable in almost all types of amplifying systems.

While I have shown and described only a few Ways in which my present invention may be carried into effect, many modifications and adaptations will be evident to those skilled in the art as falling within the spirit and scope of the invention as set forth in the appended claims.

What I claim is:

1. In an electrical signal amplifier, a vacuum tube loaded with a circuit having less than critical damping and an additional Vacuum tube for at least critically damping said circuit periodically at a rate of change high compared to the rate of change of said signals.

2. In a modulated carrier wave receiver, a source of modulated carrier signals, a demodulator, a modulation frequency amplifier connected to a modulation frequency load, a thermionic vacuum tube including input and output circuits, means for connecting said output circuit across at least a portion of said load, and means for connecting said input circuit across at least a portion of said modulated carrier source.

3. In a modulated carrier wave receiver, a source of modulated carrier signals in series with a source of demodulated signals, a thermionic vacuum tube amplifier connected between said source of demodulated signals and a modulation frequency load circuit, an additional thermionic vacuum tube including input and output electrodes, means for connecting said output electrode to said load circuit, and means for applying modulated carrier and demodulated signals from said source to said input electrode.

4. In a sloped signal amplifier, a source of sioped signals, a vacuum tube amplifier tube including input and output electrodes, a signal load circuit having less than critical damping connected to said output electrode, means for applying at least a portion of the signals from said source to said input electrode and a source of periodic signals having slopes high compared to the slopes of the first said sloped signals connected to said input electrode.

5. In an electrical wave amplifier, means for generating substantially rectangular electrical signals, means for amplifying said signals including means generating damped oscillations, and means for quenching said oscillations at a frequency high compared to the natural frequency of said oscillations.

6. In an electrical Wave amplifier, a source of signals, a loud speaker, a thermionic vacuum tube amplifier, a transformer for connecting said tube with said speaker, and means for periodically damping at least a portion of said transformer.

' ALFRED W. BARBER. 

